Reconstruction of marine redox landscape during the Cryogenian interglacial oceans using thallium isotopes

The change in marine redox chemistry between the Cryogenian Sturtian and Marinoan glaciations (ca. 663–654 Ma) is crucial for understanding the potential relationship between environmental conditions and the emergence and diversification of early animals. The Nanhua Basin in South China provides a nearly complete sedimentary record of Cryogenian interglacial sedimentation, commonly referred to as the Datangpo Formation. In this study, we present high-resolution thallium isotope (ε²⁰⁵Tl) records from the basal Datangpo Formation using two drill cores in the Nanhua Basin (South China) to constrain changes in marine redox chemistry during the Cryogenian interglacial period. Our ε²⁰⁵Tl values range from −4.9 to −0.7 with a mean of −2.8, which is lower than the upper continental crust value (ε²⁰⁵Tl_UCC = −2.1 ± 0.3) but higher than the modern seawater value (ε²⁰⁵Tl_seawater = −6 ± 0.6). We observed a negative excursion in the lower part of both drill cores and suggest an episode of ocean oxygenation in the basal Datangpo Formation. The ε²⁰⁵Tl values shifted from lower to higher values upsection in both the Gaodi and Changxingpo drill cores, suggesting a return to widespread anoxic conditions. A simple mass-balance model suggests that well-oxygenated, Mn-oxide-rich sediments acted as a significant sink for Tl in the Nanhua Basin (Gaodi: f_oxic = 7 − 14 %, Changxingpo: f_oxic = 13−20 %). Our new ε²⁰⁵Tl data also provide insights for interpreting black shale-hosted Mn-carbonate deposits in the basal Datangpo Formation, with the ε²⁰⁵Tl values from the Mn-carbonate-hosted interval supporting the transformation of Mn-carbonates from Mn-oxide precursors in the wake of Sturtian glaciation. The Tl isotope data indicate that Mn-oxide-rich sediments were a significant sink for Tl in the Cryogenian interglacial oceans; if this reflects oxygenated marine conditions, this transient oxygenation event coincides with the first significant radiation of algae, which likely laid the ecological foundation for the emergence of multicellular organisms. These findings highlight the intricate link between marine redox fluctuations and early biological evolution during the Cryogenian interglacial period.